Electronic apparatuses including smart phones, tablet terminals, digital cameras and laptop computers have DC/DC converters. FIG. 1 is a circuit diagram of a general DC/DC converter. The DC/DC converter 100R of FIG. 1 is a step-down converter (a buck converter) and includes an output circuit 102 and a control circuit 200R. The output circuit 102 includes a switching transistor M1, a rectifying transistor M2, an inductor L1 and an output capacitor C1.
The control circuit 200R includes a pulse modulator 202 and a driver 204. The pulse modulator 202 receives a detection signal VS corresponding to an output voltage VOUT of the DC/DC converter 100R, and generates a control signal S1 whose duty ratio (pulse width) is adjusted so that the detection signal VS approaches a target value. The driver 204 drives the switching transistor M1 and the rectifying transistor M2 of the output circuit 102 based on the control signal S1.
In a heavy load state where an output current IOUT is large to some extent, the DC/DC converter 100R operates in a continuous current mode (CCM). In the continuous current mode, the pulse modulator 202 enters a PWM mode and generates the control signal S1 by PWM control.
In a light load state where the output current IOUT becomes small, the DC/DC converter 100R operates in a discontinuous current mode (DCM). In the light load state, the pulse modulator 202 enters a mode (called a PFM mode) different from the PWM mode and generates the control signal S1 by PFM control.
FIG. 2 is an operation waveform diagram of the DC/DC converter 100R in a PFM mode. FIG. 2 shows a coil current IL, the state of the switching transistor M1, the state of the rectifying transistor M2, and an output voltage VOUT. The PFM mode includes an on-state TON, an off-state TOFF and a high impedance state THiZ. In the on-state TON, the switching transistor M1 is turned on, the rectifying transistor M2 is turned off, and a voltage (VIN−VOUT) is applied across the inductor L1. In the on-state TON, the coil current IL increases with a slope (VIN−VOUT)/L.
When the coil current IL reaches a predetermined peak value at time t1 or when a predetermined on-time elapses, the PFM mode transitions to the off-state TOFF. In the off-state TOFF, the switching transistor M1 is turned off, the rectifying transistor M2 is turned on, and a voltage −VOUT is applied across the inductor L1. In the off-state TOFF, the coil current IL decreases with a slope (−VOUT)/L.
When the coil current IL decrease to zero at time t2, the PFM mode transitions to the high impedance state THiZ. In the high impedance state THiZ, both the switching transistor M1 and the rectifying transistor M2 are turned off, one end of the inductor L1 is turned to high impedance, and the coil current IL is maintained at zero. In the state where the coil current IL is zero, the output capacitor C1 is discharged with a load current IOUT, and the output voltage VOUT is lowered. When the output voltage VOUT drops to a voltage VOUT(REF) defining a target value at time t3, the PFM mode returns to the on-state TON.
In the PFM mode, as the load current IOUT decreases, a rate of decrease of the output voltage VOUT in the high impedance state THiZ decreases, so that a length of the high impedance state THiZ increases. That is, as the load current IOUT decreases, the switching frequency fSW decreases, the switching loss decreases, and the efficiency can be improved.
In the PFM mode, noise problems occur in exchange for efficiency improvement. Specifically, when the switching frequency fSW enters an audible band, acoustic noise may be generated in some cases. In particular, when a ceramic capacitor is used for the output capacitor C1, the sound of the ceramic capacitor becomes a problem.
These problems may be caused in switching power supplies having different topologies of not only a step-down type but also a step-up type and a step-up/down type. The problems described here should not be understood as general recognition by those skilled in the art, but rather, they were originally recognized by the present inventors.